See PHI2009. The sshd subprocesses (bin/ssh-auth and bin/ssh-exec) currently don't have a configurable error log. These processes are somewhat unique: most other processes log to the webserver error log or the daemon log.
See also T12611.
See also T4472, perhaps.
It would be nice to better unify all log behavior, but just adding log.ssh-error.path for now is probably the more practical approach.